Before the advent of plastic containers, the amount of soft drink in a typical glass bottle was twelve fluid ounces. This amount is one or two servings, and the contents were usually completely consumed upon opening the bottle. If the contents weren't completely consumed before it went flat, the amount wasted wasn't significant for anyone to complain about.
Two and three liter bottles of soft drink have the potential problem of wasting unacceptable amounts of beverage. When the bottle leaves the bottling plant, it is pressurized at about fifteen psi as a result of the carbonation process. Upon opening the bottle, this pressure is lost, and causes the beverage to begin fizzing. With the bottle recapped, fizzing continues until the fizzing action itself repressurizes the bottle again to fifteen psi. The concentration of beverage carbonation decreases then, each time this process is repeated.
The loss of pressure above the liquid is what triggers the fizzing. Systems exist today to restore this pressure using canisters of compressed carbon dioxide. There is no system setup for the disposable plastic bottles and CO.sub.2 canisters.